lt3825 Linear Technology Corporation, lt3825 Datasheet - Page 25

• Manufacturer Part Number: lt3825
• Description: Isolated No-opto Synchronous Flyback Controller With Wide Input Supply Range
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
charging/discharging ΔV. This percentage ripple changes,
depending on the requirements of the application. You can
modify the equations below.
For a 1% contribution to the total ripple voltage, the ESR
of the output capacitor is determined by:
The other 1% is due to the bulk C component, so use:
In many applications the output capacitor is created from
multiple capacitors to achieve desired voltage ripple, reli-
ability and cost goals. For example, a low ESR ceramic
capacitor can minimize the ESR step, while an electrolytic
capacitor satisfi es the required bulk C.
Continuing our example, the output capacitor needs:
These electrical characteristics require paralleling several
low ESR capacitors possibly of mixed type.
Most capacitor ripple current ratings are based on 2000
hour life. This makes it advisable to derate the capacitor
or to choose a capacitor rated at a higher temperature
than required.
One way to reduce cost and improve output ripple is to
use a simple LC fi lter. Figure 8 shows an example of the
fi lter.
ESR
ESR
C
C
OUT
OUT
SECONDARY
COUT
COUT
WINDING
≥
≥
FROM
1% • V
1% • 5 • 200kHz
≤1% •
≤ 1% •
I
OUT
OUT
8A
V
C1
47μF
0.1μH
OUT
5V • 1– 49%
3
• f
L1
OSC
Figure 8
• 1– DC
(
(
I
8A
OUT
= 800μF
C
470μF
OUT
MAX
)
V
OUT
= 3mΩ
C
1μF
)
OUT2
3825 F08
R
LOAD
The design of the fi lter is beyond the scope of this data
sheet. However, as a starting point, use these general
guide lines. Start with a C
solution. Make C1 1/4 of C
pole independent of C
with multiple ceramic capacitors. Make L1 smaller than
the output inductance of the transformer. In general, a
0.1μH fi lter inductor is suffi cient. Add a small ceramic
capacitor (C
those interested in more details refer to “Second-Stage
LC Filter Design,” Ridley, Switching Power Magazine, July
2000, p8-10.
Circuit simulation is a way to optimize output capacitance
and fi lters, just make sure to include the component parasit-
ics. LTC SwitcherCAD ™ is a terrifi c free circuit simulation
tool that is available at www.linear.com. Final optimization
of output ripple must be done on a dedicated PC board.
Parasitic inductance due to poor layout can signifi cantly
impact ripple. Refer to the PC Board Layout section for
more details.
IC Thermal Considerations
Take care to ensure that the LT3825 junction temperature
does not exceed 125°C. Power is computed from the aver-
age supply current, the sum of quiescent supply current
(I
The primary gate drive current is computed as:
where Q
the gate charge curve) and f is the switching frequency.
Since the synchronous driver is usually driving a capacitive
load, the synchronous gate drive power dissipation is:
where C
pin max voltage.
SwitcherCAD is a trademark of Linear Technology Corporation.
CC
f
f
OSC
OSC
in the specifi cations) plus gate drive currents.
• Q
• C
G
S
is the total gate charge at max V
is the SG capacitive load and V
S
G
OUT2
• V
SGMAX
) for high frequency noise on V
OUT
OUT
. C1 may be best implemented
OUT
1/4 the size of the nonfi lter
to make the second fi lter
GS
SGMAX
(obtained from
LT3825
is the SG
OUT
25
. For
3825fa